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MG Mgb Problems & Known Issues

The most common MG Mgb problems ranked by severity — what to watch for, what it costs to fix, and what happens if you ignore it.

Generations

MkI — MGB Mark I
1962–1967
B-Series 1.8
MkII — MGB Mark II
1967–1969
B-Series 1.8
MkIII — MGB Mark III
1969–1974
B-Series 1.8, B-Series 1.8
MkIV — MGB Mark IV (Federal bumpers)
1974–1980
B-Series 1.8

Top issues to watch for

The most serious MG Mgb problems ranked by severity. We track 5 known issues — here are the ones you need to know about first.

Dangerous
Extensive body corrosion
MkI, MkII, MkIII, MkIV18GB, 18GD, 18GF1962–1980
The MGB monocoque construction uses thin steel panels that corrode extensively in the British climate. Critical structural areas include the sills (which form part of the chassis), floor pans, A-post bases, and rear spring hangers. Unlike modern cars with separate chassis, MGB structural integrity depends entirely on the shell remaining sound. Many examples have been poorly repaired with fibreglass or filler, masking serious structural weakness. Professional restoration costs often exceed the finished car's value.
What happens if ignoredSill corrosion renders the car structurally unsound and dangerous to drive. Floor rust-through allows exhaust fumes into the cabin. Professional steel sill replacement costs £2,000-£5,000 plus paintwork. Many MGBs are written off rather than properly repaired due to the extent of hidden corrosion discovered once restoration begins.
Independent garage
£500–£2,000
Specialist
£2,000–£8,000
Parts only
£200–£1,000
Major
Overdrive unit failure
MkII, MkIII, MkIV18GD, 18GF, 18V1967–1980
The Laycock de Normanville Type A overdrive unit fitted to many MGBs operates hydraulically using engine oil pressure. The system includes solenoids, one-way clutches, and precision-machined components that wear over time. Electrical switch failure, worn solenoids, or internal clutch wear can prevent engagement or cause the unit to slip under load. Repair requires specialist knowledge and is expensive because the unit must be removed with the gearbox.
What happens if ignoredComplete overdrive failure leaves the car with a standard four-speed gearbox. Slipping overdrive makes motorway driving uncomfortable due to high engine speeds in fourth gear. Specialist rebuild costs £800-£1,500 plus removal and refitting. Some owners choose to live without overdrive rather than repair it.
Minor
Lucas electrical system failures
MkI, MkII, MkIII, MkIV18GB, 18GD, 18GF1962–1980
MGB electrical systems use Lucas components including the notorious positive-earth charging system (pre-1967) and various switches, relays, and wiring looms of variable quality. Common failures include indicator switches, headlight relays, charging system irregularities, and general wiring deterioration. The reputation for unreliability is partly historical but many surviving cars have modified or updated electrical systems that may not be properly documented.
What happens if ignoredElectrical failures can leave you stranded or create safety issues if lights fail at night. Most faults are repairable by any auto-electrician familiar with classic cars. Complete rewiring costs £500-£1,500 but dramatically improves reliability. The car remains drivable with most electrical faults.
Independent garage
£100–£400
Specialist
£150–£500
Parts only
£20–£150
Minor
Twin SU carburettor imbalance and wear
MkI, MkII, MkIII, MkIV18GB, 18GD, 18GF1962–1980
All MGBs use twin SU carburettors (HS4 or HIF types depending on year) that require regular balancing to maintain smooth running. The carburettors have adjustable mixture screws, throttle synchronisation, and float chambers that can drift out of specification. Modern ethanol-blend petrol can affect rubber diaphragms and gaskets more than period fuel. Worn carburettors cause poor fuel economy, flat spots during acceleration, and rough idling.
What happens if ignoredPoor carburettor setup reduces performance and fuel economy but does not damage the engine. Professional balancing and tune costs £100-£200 and should restore smooth running. Complete carburettor rebuilds are needed every 20,000-30,000 miles depending on fuel quality and usage patterns.
Independent garage
£80–£150
Specialist
£120–£250
Parts only
£30–£80

1 more known issues tracked

Including wear items, electrical niggles, and generation-specific problems.

MOT failure hotspots

Based on DVSA MOT test data. These are the items where the MG Mgb fails at a significantly higher rate than the average car.

Wheel bearings worn
Suspension · 0-20k miles
MOT failure rate: 37x the fleet average
Sample: 139 tests
Fluid leak
Exhaust & Emissions · 20-40k miles
MOT failure rate: 17x the fleet average
Sample: 225 tests
Brake pipes corroded
Brakes · 20-40k miles
MOT failure rate: 8.3x the fleet average
Sample: 109 tests
Suspension pin, bush or joint worn
Suspension · 0-20k miles
MOT failure rate: 7.0x the fleet average
Sample: 152 tests

Looking at a specific MG Mgb?

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Related

MG Mgb reliability FAQs

What are the most common problems with the MG Mgb?

The MG Mgb has 5 known issues across 3 severity levels. The most serious are: Extensive body corrosion.

Is the MG Mgb reliable?

The MG Mgb has 1 dangerous and 1 major known issues. Most can be avoided with a thorough pre-purchase check and full service history.

How much does it cost to fix MG Mgb problems?

Repair costs range widely. The most significant issue (Extensive body corrosion) costs £500–£2,000 at an independent garage. Minor issues can be under £200. Always get a full inspection before buying.

Which MG Mgb generation is most reliable?

The MkI (1962–1967) has the fewest dangerous issues (1). However, condition and service history matter more than generation — a well-maintained earlier car can be more reliable than a neglected newer one.